"A Pig in a Poke"

A pig in a poke is an item or an idea that seems to be one thing but might well contain something different. In medieval England, small pigs were sold at market in pokes, or little bags. At times, however, some crafty farmers took to putting a cat in the bag and tried to sucker someone into buying the pig without looking inside the poke. If they looked, the farmer would warn, the pig might run away. Those skeptics who did look invariably "let the cat out of the bag."